Movement results from the complex interplay between neural and musculoskeletal systems. Whether investigating motor control in healthy subjects or following injury, both systems must be considered in order to interpret the function, or dysfunction, of movement. Our long term research goal is to investigate this interplay, providing insights into the mechanisms and strategies underlying biological motor control in health and disease. In the research proposed here, we will examine the production of locomotion in the rat, examining both its neural control and its biomechanics. Although the rat is being used increasingly to study motor control and the consequences of injury, many features of its behavior and biomechanics are unknown. We will evaluate a specific hypothesis about biological motor control: that motor systems produce movement through the flexible combination of a small number of muscle groups, or muscle synergies. We propose that the muscles within each such group are not arbitrary but are adapted to the biomechanics of the motor system. Our specific aims are to 1) Evaluate, using novel computational analyses, whether the patterns of muscle activations recorded during locomotion in freely behaving rats can be well described as the combination of muscle synergies;2) Develop a biomechanical model of the hindlimb musculoskeletal system of the rat to be used in interpreting the identified muscle synergies;3) Use this model to examine the biomechanical actions of identified synergies and to assess whether complex behaviors can be produced by combination of muscle synergies. The research proposed here thus serves two simultaneous purposes, both of potential relevance to public health. First, by testing this specific hypothesis of the production of complex movement by a mammal, this research will provide insights to motor control in other mammals, including humans. Second, by providing basic information and powerful computational tools for the analysis of motor control in the rat, this research will greatly increase our basic understanding of this important model system. Based on this understanding, we can better evaluate the effects of injury in this system and can more readily develop strategies of rehabilitation and regeneration, strategies which might then be translated into clinical settings.